The present invention relates to centrifugal pumps in general, and more particularly to improvements in so-called chemical centrifugal pumps or tank farm pumps for handling of liquid chemicals, especially liquid chlorine.
In accordance with heretofore known proposals, liquid chlorine is pumped by resorting to canned motor pumps without shaft seals, magnetic coupling pumps or conventional wet-mounted shaft sump pumps with the shaft sealed leaktight at its passage out of the tank. In each embodiment, the rotor is mounted in one, two or more submerged bearings which are lubricated by the circulated liquid. In order to avoid corrosion, chlorine is often circulated at elevated temperatures such as to exclude icing, misting or fogging of component parts of the plant. However, this produces certain undesirable side effects, such as a greatly reduced lubricating action of chlorine as well as a greatly increased chemical aggressiveness of chlorine upon various bearing materials. This, in turn, greatly reduces the useful life of the bearings which are used in such types of pumps. An additional drawback of handling of liquid chlorine at elevated temperatures is that, in a canned motor pump, heat which is generated by the motor must be led away by a partial liquid stream which is intended for the lubrication of bearings; this brings about additional undesirable effects upon the lubricating qualities of liquid chlorine and further reduces the useful life of the bearings.
In machines of the just outlined character, pronounced wear upon the bearings entails metallic contact between rotary and stationary parts, especially between rotary parts and portions of the housing. The resulting friction entails the development of elevated temperatures which can lead to combustion of chlorine. It will be readily appreciated that pronounced friction is especially damaging in narrow clearances between races of bearings and adjacent ring-shaped parts as well as, in the case of canned motor or magnetic coupling pumps, between the rotor and the stator. If metallic contact is not detected without delay and the pump is not arrested in immediate response to such detection, one risks the development of cracks in the walls which allow the escape of chlorine into the surrounding area. Monitoring of wear upon the bearings cannot eliminate the just discussed problems because leaks can develop in the stator can (whose thickness is in the range of a few tenths of one millimeter), for example, as a result of penetration of foreign matter which remains entrapped in the narrow clearance.
Attempts to prevent escape and/or burning of chlorine include other expensive proposals, for example, the utilization of detectors which furnish signals in response to penetration of chlorine into the interior of the stator. Furthermore, it is necessary to manufacture all such parts which are subjected to elevated pressures from special materials whose combustion temperature in chlorine is relatively high. Suitable materials of such character include nickel alloys or other substances having a relatively high nickel content which contributes excessively to the initial cost of the machine.